Heretofore, there has been a printing machine including a transfer mechanism for transferring record sheets using an endless transfer belt. In this printing machine, record sheets are transferred using the transfer belt and are sequentially moved to pass through multiple ink heads which are arranged in the direction of transfer thereof and configured to form images of different single colors, respectively. This enables a color image to be obtained by superimposing images of the respective single colors on a record sheet.
Meanwhile, highly-accurate drive control for moving the transfer belt at a constant travel speed is required. For this reason, as a mechanism for keeping a constant rotational speed of a drive roller configured to drive the belt, there have heretofore been known drive control methods for controlling the rotation of the drive roller. Such drive control methods include one by which the rotational speed of the drive roller is kept constant by keeping constant the angular speed of a motor, which serves as a drive source, and the angular speed of a gear, which is configured to transmit the rotational driving force generated by the motor to the drive roller.
However, a variance in the belt thickness in the circumferential direction of the belt exists; therefore, there is the problem that the travel speed of the belt changes due to this variance. This belt thickness variance is caused by a deviation in wall thickness in the circumferential direction of the belt, and is observed in a belt fabricated by, for example, centrifugal sintering using a cylinder mold. In the case where such a belt thickness variance exists in the belt, the belt travel speed is high when a portion of the belt which has a large thickness is placed around a drive roller which is configured to drive the belt, and, on the other hand, the belt travel speed is low when a portion of the belt which has a small thickness is placed around the drive roller. Thus, a variation occurs in the belt travel speed.
In the case where the travel speed of the transfer belt is not kept constant as described above, when single-color images are to be formed on a record sheet respectively using multiple ink heads, and these images of multiple colors are to be superimposed on each other, so-called “ink misalignment” occurs in which the respective transfer positions of the single-color images are misaligned relative to each other. If such an ink misalignment occurs, a thin line image formed by superimposing images of multiple colors on each other may look blurred, and a white spot may appear around the outline of a black character image formed in a background image which is formed by superimposing images of the multiple colors, for example.
As a technique for a reducing belt speed variation to prevent such an ink misalignment, for example, there is a technique described in Patent Document 1. In this technique disclosed in Patent Document 1, a thickness profile (belt thickness variance) over the entire loop of the belt is measured in advance, and data on the thickness profile is stored in data storage. Then, the phase of the thickness profile data for the entire loop and that of actual belt thickness variance are matched to each other, and print timings are changed so that print positional deviation due to the belt speed variation may not occur.
Specifically, in this technique disclosed in Patent Document 1, from data on the difference between the angular velocities of two rollers (a drive roller and a driven roller) over which a transfer belt is passed, an alternating current component of the angular speed which has a frequency corresponding to a belt speed variation is extracted. From data on the amplitude and phase of the alternating current component thus extracted, a belt speed variation due to the belt thickness variance is recognized. Based on the belt speed variation thus recognized, the timing for the initiation of image formation and the speed of image formation during the image formation are adjusted for each of the multiple images.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 2006-227192